The adaptive immune response to mucosal pathogens is still poorly understood, limiting the development of new vaccines for important human pathogens such as the category B bioterrorism agent Salmonella. We propose to use newly developed reagents to examine the priming of Salmonella-specific CD4 T cells in the intestinal mucosa and systemic tissues, explore the mechanism of effector cytokine production in the infected liver, and examine the effect of bacteria on the survival of Salmonella- specific T cells in vivo. The specific aims of this proposal are: Aim 1. Test the hypothesis that bacterial antigen expression shapes the Salmonella-specific CD4 repertoire at mucosal and systemic sites. Aim 2. Test the hypothesis that innate activation of Salmonella-specific CD4 T cells amplifies effector cytokine production in infected tissues. Aim 3. Test the hypothesis that live Salmonella can inhibit survival of activated CD4 T cells via SPI2 effector proteins. Our preliminary data demonstrate that we have identified several new targets of Salmonella-specific CD4 T cells and can develop novel MHC class-II tetramer reagents to track the mucosal and systemic Salmonella-specific T cell response in vivo. Our data also demonstrate that previously activated CD4 T cells can be activated by innate stimuli without TCR ligation. Lastly our preliminary data indicate that live bacteria inhibit the survival of Salmonella-specific T cells in vivo. Our three specific aims will use cutting-edge technology, most of which has been developed by our laboratory during the previous funding cycle, to examine the priming, effector function and survival, of Salmonella-specific CD4 T cells throughout the course of Salmonella infection.